The present invention relates generally to a read back circuit (also known as a "read chain" circuit) for recovering data from a storage medium or a transmission line. In particular, the present invention has application to read back circuits that utilize a differentiation technique for detecting peaks in recovered data. More particularly, the present invention is directed to an anti-shouldering circuit for use with dual density magnetic disk drives of the type that utilize a differentiator to detect peaks in the read signal.
Known read chain circuits include a read head, a read head preamplifier, a low pass filter (analog), a differentiator, a comparator and a time domain filter (digital). The read head preamplifier amplifies the signal from the read head to usable levels. The low pass filter reduces noise in the read circuit. The differentiator differentiates the read signal from the low pass filter and thus provides zero crossovers in time coincidence with peaks in the read signal. Thus, zero crossovers in the differentiated read signal are indicative of peaks, and hence data, in the read signal. The comparator squares the differentiated read signal into logic levels; changes in the logic levels provided by the comparator correspond to peaks in the read signal, and hence to data. The squared read data from the comparator is then supplied to a time domain filter and to a read data one shot circuit.
Various encoding techniques for storing digital data on a rotating magnetic medium, such as a floppy disk, are known. These include FM and MFM encoding techniques. Presently, it is customary to use a MFM encoding technique in dual density disk drives. Typically, the high density 2 F frequency is 250 KHz, the high density 1 F and low density 2 F frequencies are 125 KHz, and the low density 1 F frequency is 62.5 KHz.
A known problem with differentiator circuits for dual density disk drives is "shouldering" i.e., the differentiator output may "droop" between the occurrence of sequential peaks in the read signal. If the time between sequential peaks is long, as is common when low frequency patterns (e.g., low density 1 F data) are recorded on the outermost tracks of a disk, the differentiator output may "droop" so far as to provide an erroneous zero crossover, and hence erroneous data.
Moreover, known read chain circuits for dual density drives require two sets of filters to recover both high and low density data. This is due primarily to the design of the time domain filter. The typical time domain filter is a one shot circuit that disables (delays) the read data for a predetermined period of time after a zero crossover in the differentiated read signal has been detected. The time domain filter will tolerate only a one octave frequency span, but frequency spans as wide as two octaves (i.e., the span between high density 2 F data and low density 1 F data) are found in dual density drives. Thus, two separate time domain filters are required, i.e., one for high density data and one for low density data.
The present invention is directed to a read chain circuit that eliminates erroneous data due to shouldering and does not require a time domain filter.